Direct viewing bistable storage tube having fast erase speed

ABSTRACT

A direct viewing bistable storage tube is described having a storage dielectric layer of phosphor material supported on the glass faceplate of such tube with a light transparent target electrode provided beneath such phosphor layer and a collector electrode in contact with the opposite side of such phosphor layer. High image resolution results from providing the collector electrode as a very fine mesh coating on the phosphor layer. The target electrode may be used as an erase electrode and because it is separate from the collector electrode the tube is provided with a faster erase speed, of about 1 to 50 milliseconds. Thus, the voltage on the collector electrode can be set for optimum secondary electron collection during erasure, as well as during storage, while the voltage of the erase electrode can be varied to provide an erase pulse of the proper amplitude and time for optimum erasure. The erase electrode can be split into a plurality of insulated conductive areas to provide independent operation of different portions of the storage dielectric in a storage or nonstorage mode. An intermediate layer of light transparent insulating material, such as silicon dioxide, may be provided between the target electrode and the phosphor layer to increase the voltage breakdown strength of the dielectric.

United States Patent 3,368,093 2/1968 Sjoeberg Inventor Roger A. Frankland Portland. Oreg.

Appl. No. 27,383

Filed Apr. 10, I970 Patented July 20, 197] Assignee Tektronix, lnc.

Beaverton. Oreg.

Continuation-impart of application Ser. No. 8l6,576, Apr. 16, I969.

DIRECT VIEWING BISTABLE STORAGE TUBE Primary Examiner-Rodney D. Bennett, Jr Assistant Examiner-Joseph G. Baxter Attorney-Buckhorn, Blore, Klarquist & Sparkman ABSTRACT: A direct viewing bistable storage tube is described having a storage dielectric layer of phosphor materia] supported on the glass faceplate of such tube with a light transparent target electrode provided beneath such phosphor layer and a collector electrode in contact with the opposite side of such phosphor layer. High image resolution results from providing the collector electrode as a very fine mesh coating on the phosphor layer. The target electrode may be used as an erase electrode and because it is separate from the collector electrode the tube is provided with a faster erase speed, of about 1 to 50 milliseconds. Thus, the voltage on the collector electrode can be set for optimum secondary electron collection during erasure, as well as during storage, while the voltage of the erase electrode can be varied to provide an erase pulse of the proper amplitude and time for optimum erasure. The erase electrode can be split into a plurality of insulated conductive areas to provide independent operation of different portions of the storage dielectric in a storage or nonstorage mode. An intermediate layer of light transparent insulating material, such as silicon dioxide, may be provided between the target electrode and the phosphor layer to increase the voltage breakdown strength of the dielectric.

7O Q UT ROGER A. FRANKLAND INVENTOR ATTORNEYS PATENTED JUL 20 um FIG. I5

I g v FIG. 2

48 ERASE PULSE VER AMP GEN.

BUCKHORN, BLORE, KLARQUIST & SPARKMAN DIRECT VIEWING BISTABLE STORAGE TUBE HAVING FAST ERASE SPEED CROSS REFERENCE TO RELATED APPLICATION The present application is a continuation-in-part of copending U.S. Pat. application Ser. No. 8l6,576, filed Apr. 16, 1969, by R. A. Frankland, for BISTABLE STORAGE TUBE HAVING FAST ERASE SPEED.

BACKGROUN D OF THE INVENTION The present invention relates generally to storage tubes which are capable of bistable storage of charge images and, in particular, to direct viewing bistable storage tubes employing a storage dielectric of phosphor material coated on the glass faceplate of such tube over a light transparent target electrode, as shown in U.S. Pat. No. 3,214,631, granted Oct. 26, 1965, and U.S. Pat No. 3,293,473, granted Dec. 20, 1966, both of R. H. Anderson. The present storage tube is an improvement over previous tubes of this type in that it employs a separate collector electrode supported in contact with the opposite side of the phosphor layer from the target electrode. As a result, the target electrode can operate as an erase electrode by varying the voltage of the storage dielectric without changing the voltage on the collector electrode so that such collector voltage can be maintained at the optimum value for secondary electron collection. This provides the present bistable storage tube with an extremely fast erase speed of about I to 5 milliseconds which is approximately lOO times faster than previous tubes of this type.

The storage tube of the present invention is especially useful as a direct viewing bistable storage tube which can serve as the display tube of a cathode-ray oscilloscope because it has a phosphor storage dielectric which emits a light image corresponding to the stored charge image. However, such tube may also be used as a scan converter tube having electrical readout by providing means for scanning an electron beam across the storage dielectric to produce an electrical readout signal corresponding to the charge image, in which case a storage dielectric other than phosphor material may be employed.

While collector electrode meshes have previously been employed in bistable storage tubes, such as that of British Pat. No. 48l,094, such meshes are supported in spaced relationship to the storage dielectric and, as a result, must be a relatively thick self-supporting structure which limits the image resolution of the stored charge image. Barrier-grid-type storage tubes which are not capable of bistable storage employ a similar target structure but for an entirely different purpose, as shown in U.S. Pat. No. 3,l8l,02l ofC. L. Day, issued Apr. 27, 1965. In these latter storage tubes, the mesh electrode which contacts the storage dielectric does not function as a collector for the secondary electrons, but rather merely as a barrier grid to prevent the secondary electrons from returning to the storage dielectric by repelling them to a separate collectorelectrode. In addition, erase pulses are not applied to the target electrode, but rather erasure is accomplished by scanning the target with an electron beam emitted from the writing gun cathode at a more negative voltage than that used during writing, as shown in U.S. Pat. No. 2,943,231 of Boulet et al. issued June 28, I960. Thus, the operation of a barrier grid storage tube is completely different from that of the bistable storage tube of the present invention and such barrier grid storage tubes do not employ flood guns for uniform bombardment of the target with low-velocity electrons to enable bistable storage of a charge image.

In addition to its fast erase speed, the present storage tube has the advantage that it is capable of extremely high image resolution due to the fact that the collector mesh electrode is provided as a coating to form an extremely thin, fine mesh. Furthermore, by dividing the erase electrode into a plurality of separate insulated conductive areas and selectively applying erase voltages to such areas, different portions of the storage dielectric over such conductive areas may be operated independently in a storage or nonstorage mode to provide a more versatile tube. Thus, by continuously applying erase pulses at a frequency of 60 Hertz or more to prevent flicker, either of the storage dielectric portions can be operated in a nonstorage mode. This split screen storage tube has no objectionable shadowing around the edges of the phosphor screen or along the split or gap between adjacent conductive areas, as happens when a split collector electrode is employed.

As a result of the fast erase speed and good bistable operation, the present storage tube, when operated as a scan converter storage tube, produces an electrical readout signal of good quality and fast repetition rate. Furthermore, since the collector electrode mesh is made of a light reflecting metal, it increases the brightness of the light image emitted by the phosphor by approximately 50 percent for conventional nonstorage operation. Thus, when its phosphor is 50 percent aluminized by the collector electrode, the present tube produces a light image having a brightness of about 75 percent that of a fully aluminized phosphor screen. Another advantage of one embodiment of the present storage tube employing an additional intermediate layer of insulating material having a higher dielectric constant than the phosphor layer is a greater usable production yield and greater reliability for such tube as well as a wider operating voltage range due to the higher voltage breakdown strength of the dielectric.

It is therefore one object of the present invention to provide an improved bistable storage tube of extremely fast erase speed.

Another object of the present invention is to provide a bistable storage tube having a mesh collector electrode coated on the storage dielectric to provide a fine mesh which enables high image resolution.

A further object of the present invention is to provide an improved direct viewing bistable storage tube having a storage dielectric of a phosphor material provided on a light transparent erase electrode and a collector mesh electrode of lightreflecting material coated over such phosphor layer to increase the brightness of the light image emitted thereby to provide such image with a high resolution and to provide the storage tube with a fast erase speed.

An additional object of the invention is to provide such a direct viewing bistable storage tube with greater versatility by dividing the erase electrode into a plurality of separate insulated areas to enable independent operation of the portions of the phosphor layer above such areas in a storage mode or a nonstorage mode with no undesirable shadowing around the edges of the phosphor layer or adjacent the gap between erase electrode areas.

A still further object of the present invention is to provide a bistable storage tub capable of electrical readout with extremely fast erase speed, high resolution and good bistable storage operation so that such tube can function as a scan converter for high-speed data transmission.

Still another object of the invention is to provide such a direct viewing bistable storage tube with an intermediate layer of light transparent insulating material between the phosphor layer and the target electrode to increase the voltage breakdown strength of the dielectric and thereby provide a greater production yield and reliability for the tube as well as increasing its operating voltage range.

BRIEF DESCRIPTION OF THE DRAWINGS Other objects and advantages will be apparent from the following description of a preferred embodiment of the invention and from the attached drawings of which:

FIG. 1 is a schematic view of one embodiment of a storage apparatus employing the direct view bistable storage tube of the present invention;

FIG. 2 is an elevation view taken along theline 2-2 of FIG. 1 showing the split erase electrodes and the lead portion of the collector electrode coated on the tube faceplate FIG. 3 is a horizontal section view taken along the line 3-3 of FIG. 2; and

FIG. 4 is a partial section view of another embodiment of the storage target of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT As shown in FlG. l, the storage apparatus of the present invention includes a direct viewing bistable storage tube having a split screen" type storage target similar to that shown in U.S. Pat. No. 3,214,631 of R. H. Anderson. The tube includes a storage target 12 supported on the light transparent glass faceplate 14 forming part of the evacuated tube envelope hereafter described in greater detail with respect to FIG. 3. The storage tube includes a conventional writing electron gun whose cathode 16 may be connected to a negative DC supply voltage of about 3,000 volts. The writing gun also includes a control grid 18, a focusing and accelerating anode structure 20, horizontal deflection plates 22 and vertical deflection plates 24. When the storage tube is used in a cathode-ray oscilloscope, the input signal is applied to the vertical deflection plates 24 through a vertical amplifier 26 while a ramp voltage sweep signal is applied to the horizontal deflection plates 22 by a sweep generator 28. The sweep generator may be triggered in response to the receipt of a vertical input signal at input terminal 30 by transmitting a portion of such input signal to a trigger generator 32 whose output is connected to such sweep generator.

A charge image is written on the storage dielectric of the target 12 by the high-velocity electron beam of the writing gun. 1f the potential of such charge image exceeds the critical first crossover voltage on the secondary electron emission characteristic curve of such storage dielectric, such charge image may be stored bistably in a conventional manner by uniform bombardment of the target with low-velocity flood electrons. The flood electrons are emitted from a pair of flood guns having cathodes 34, control grids 35, and focusing anodes 36. The flood gun cathodes 34 are grounded. A plurality of suitable collimating electrodes may be provided to cause the flood electrons to strike the storage dielectric at right angles thereto. One such collimating electrode 38 is shown as a wall band coated on the inner surface of the tube envelope and connected to a source of positive DC supply voltage of approximately +50 volts.

As shown in FIGS. 2 and 3, the storage target 12 includes a storage dielectric 40 which, for a direct viewing bistable storage tube, may be a continuous or undivided layer of phosphor material, such as manganese activated zinc orthosilicate, referred to as P1 phosphor, coated on the inner surface of the faceplate 14. A target electrode 42 is provided beneath the phosphor layer 40 in the form ofa light transparent film of conductive material, such as tin oxide, coated on the inner surface of the faceplate 14. The faceplate 14 is a flat plate of light transparent glass which serves as the support member for the storage target. An electron permeable collector electrode 44 is positioned in contact with the bombarded surface of the phosphor layer 40 on the opposite side of such layer from the erase electrode 42. The collector electrode 44 may be a mesh electrode of light-reflecting material, such as aluminum, coated on the phosphor layer by vapor deposition through a suitable mask. The resulting collector mesh which may be 200 lines per inch with an electron transmission of 80 percent. Thus, in this 80 percent transmission embodiment, the phosphor is only 20 percent aluminized by the collector electrode. in order to increase the brightness of the light image emitted by a trace during conventional nonstorage operation to 75 percent of the brightness of a fully aluminized tube, the phosphor should be 50 percent aluminized by the mesh electrode. it should be noted that other materials may be employed for the electrodes 42, 44 and support member 14 if the storage dielectric layer 40 is of a nonphosphor storage dielectric material, such as aluminum oxide, since then the storage tube is only capable of electrical readout.

As shown in FIG. 3, a collector lead 46 of light transparent tin oxide is coated on the inner surface of the glass faceplate l4 insulatingly spaced from the erase electrode 42 and electri cally connected to the collector electrode 44 in order to apply a DC operating voltage of about volts to such collector electrode. This enables the collector electrode 44 to collect secondary electrons emitted from the storage dielectric layer 40 due to bombardment by primary electrons emitted from either the writing gun cathode 16 or the flood gun cathode 34, since such collector electrode is highly positive with respect to such cathodes.

The target electrode 42 may be split or divided into two spaced insulated conductive portions 42A and 42B which effectively form two independently operated target electrodes in order to provide a split screen" storage target of the type discussed in U.S. Pat. No. 3,214,631 mentioned above. The target electrodes 42A and 428 may be connected to different DC operating voltages, respectively provided by a pair of voltage dividers 4850 and 52-54. Each of the voltage dividers includes a variable resistor 50 and 54 to change the value of the quiescent DC operating voltage applied to the target electrodes, although for erasure such voltage will be 0 volt or the same as the flood gun cathode voltage. in addition, an erase pulse generator 56 having two different outputs is provided with such outputs connected to the target or erase electrodes 42A and 423 so that the portions of the phosphor storage dielectric 40 above such erase electrodes can be independently erased and operated in a storage mode or a nonstorage mode. The nonstore operation is provided by continuously pulsing the erase electrode at a repetition rate of about 60 Hertz or above the flicker rate. This continuous erase pulsing is made possible by the extremely fast erase speed.

in addition, it would also be possible to provide independent storage and nonstorage operation of different portions of the phosphor layer 40 by splitting the collector mesh electrode 44 into two spaced, insulated collector electrodes to which different voltages may be applied during the formation of the charge images. However, this has the disadvantage that shadowing could occur around the edge of the phosphor layer and in the phosphor region below the gap between such collector electrodes because the flood electrons are deflected due to the coplanar grid effect."

As shown in FIG. 3, the storage tube envelope includes a funnel portion 58 of crystalline ceramic material, such as Fosterite, which is sealed to the glass faceplate 14 by an intermediate seal portion 60 of crystallized glass material or a glass different from that of the faceplate, as disclosed in U.S. Pat. No. 3,207,936 of W. H. Wilbanks et al., issued Sept. 21, 1965. The tin oxide coatings forming the lead portion 46 of the collector electrode and the lead portions and the left ends of the erase electrodes 42A and 42B, extend through the seal between the seal portion 60 and the faceplate 14 out to the exterior of the tube envelope where they are connected to the external voltage sources. Thus, the collector lead portion 46 is connected to a voltage divider including a fixed resistor 62 and a variable resistor 64 connected in series between a source of positive DC supply voltage +V and ground, and such variable resistor is adjusted to provide a DC operating voltage of about +170 volts on the collector electrode. The DC operating voltage applied by the voltage dividers 48-50 and 52-54 to the erase electrodes 42A and 428, respectively, is normally set at the zero voltage of the flood gun cathode.

The erase pulses applied to the erase electrodes by the erase pulse generator 56 may be in the form of rectangular pulses 66 having an amplitude of +200 volts and a pulse width of about 1 to 5 milliseconds. This erase pulse rapidly drives the potential of the storage dielectric above the first crossover voltage and then the flood electrons cause the entire surface of such storage dielectric to "fade positive to a uniform voltage approximately equal to the +170 volts on the collector electrode, thereby erasing the charge image. When the erase pulse returns to zero voltage, the potential of the storage dielectric follows due to capacitive coupling to the erase electrode until it drops below the first crossover voltage, and then the flood electrons cause such potential to return to a uniform voltage approximately equal to the voltage of the flood gun cathode, thereby conditioning the target for another writing operation.

An electrical readout signal can be produced on the collector electrode 44 by employing the writing electron gun 16 as a reading gun to cause the electron beam to uniformly scan the surface of the storage target. This is achieved in a conventional manner by connecting the horizontal and vertical deflection plates to the ramp signal outputs of a television raster generator (not shown). As a result of this scanning operation, an electrical readout signal corresponding to the charge image stored on the storage dielectric is transmitted to an output terminal 68 connected to the junction of the voltage divider resistors 62 and 64 through an AC coupling capacitor 70. This output signal then can be applied to the Z-axis input of a television monitor to vary the brightness of the electron beam which is scanned in a rectangular raster pattern at the same rate as the reading beam of the storage tube to provide the necessary synchronization.

It should be noted that, while the phosphor storage dielectric layer 40 is a continuous undivided layer of phosphor to provide best image resolution and to enable the collector electrode 44 to be coated thereon, this is not essential. Thus, it is also possible to provide the phosphor layer 40 in the form of a plurality of spaced islands or dots of phosphor. In addition, rather than two horizontal extending erase electrodes, it is also possible to provide an extremely large number of vertical erase electrode strips which may be sequentially pulsed to provide erasure immediately before the next charge image is written thereon, thereby effectively providing a controlled persistence storage. With such controlled persistence storage, charge image information is stored and displayed until the arrival of new information, at which time it is automatically erased and the new information is stored and displayed.

As shown in FIG. 4, another embodiment of the storage tube of the present invention includes a modified storage target I2' but is otherwise similar to the tube of FIGS. 1 to 3. The modified storage target 12 differs from the target 12 of FIG. 3 by employing an additional intermediate layer 72 of light transparent insulating material having a higher dielectric constant than the phosphor material in the storage dielectric layer 40. The intermediate insulating layer 72 may be a nonporous layer of fused silicon dioxide or other glass and is provided with a uniform thickness, of between about 5,000 and 9,000 Angstroms, by conventional vapor deposition techniques on the faceplate 14 between the phosphor layer 40 and the target electrode 42. As a result of the intermediate insulating layer 72, the voltage breakdown strength of the dielectric between electrodes 42 and 44 is increased, thereby providing a bistable storage tube of greater reliability and greater production yield which may be operated over a greater range of operating voltages. Thus, the quiescent DC voltage applied to the target electrode 42 during writing and storage may be about +70 volts and a rectangular erase pulse 74 of +280 volts amplitude and 2 milliseconds width is applied to such target electrode by the erase pulse generator 56 to increase its voltage from +70 to +350 volts and erase the charge image stored on the phosphor layer.

It will be obvious to those having ordinary skill in the art that many changes may be made in the above-described preferred embodiment of the present invention without departing from the spirit of the invention. Therefore, the scope of the present invention should only be detennined by the following claims.

lclaim:

l. A storage apparatus capable of bistable storage of a charge image. comprising:

a storage target including a support member of electrical insulative material, a storage dielectric layer of phosphor material provided on said support member, and a target electrode provided of said support member between the dielectric layer and said support member;

writing means for bombarding the storage dielectric with a beam of high-velocity electrons to'form a charge image thereon;

holding means for bombarding the phosphor storage dielectric with low-velocity electrons and producing secondary electron emission to cause bistable storage of said charge image; and

a collector electrode supported in contact with the phosphor dielectric layer on the opposite side of said dielectric layer from said target electrode so that said collector electrode is electrically insulated from said target electrode and positioned to collect the secondary electrons.

2. Storage apparatus in accordance with claim I in which the storage dielectric layer contains phosphor material which emits a light image corresponding to the charge image, and the support member and target electrode are both light transparent.

3. Storage apparatus in accordance with claim 2 in which the storage target also includes an intermediate layer of light transparent insulating material having a dielectric constant greater than that of the phosphor material, said intermediate layer being provided on the support member between the phosphor layer and the target electrode.

4. Storage apparatus in accordance with claim I in which the collector electrode is a mesh electrode formed by a coating of electrical conductive material on the phosphor layer.

5. Storage apparatus in accordance with claim 2 in which the target electrode is divided into a plurality of spaced insulated conductive areas, and which also includes means for selectively applying erase pulses to different ones of said conductive areas to enable independent operation of the storage dielectric portions over said conductive areas in a storage mode or a nonstorage mode.

6. Storage apparatus in accordance with claim 2 provided by a cathrode-ray storage tube in which the storage target and the collector electrode are contained within the evacuated envelope of said tube.

7. Storage apparatus in accordance with claim 6 in which a flat glass faceplate portion of the envelope forms the support member of the storage target and is sealed to a ceramic funnel portion of the envelope by an intermediate seal portion, and said target electrode and said collector electrode are connected to external voltage sources outside of the envelope by spaced, insulated leads of electrical conductive material coated on the faceplate beneath the seal portion and extending completely through the seal.

8. Storage apparatus in accordance with claim 7 in which the leads are made of tin oxide material and the seal portion is made of a crystallized glass material.

9. Storage apparatus in accordance with claim 4 in which the storage dielectric is a continuous undivided layer of phosphor material.

10. Storage apparatus in accordance with claim 9 in which the collector electrode is made of light-reflecting metal to increase the brightness of the light image.

11. Storage apparatus in accordance with claim 1 which also includes electrical readout means for scanning the storage dielectric with an electron beam to produce an electrical readout signal of the charge image.

12. Storage apparatus in accordance with claim 1 which also includes erase means for applying a voltage pulse to said target electrode to erase said charge image.

13. Storage apparatus in accordance with claim 12 in which the storage dielectric layer contains phosphor material which emits a light image corresponding to the charge image, and the support member and target electrode are both light transparent.

14. Storage apparatus in accordance with claim 13 in which the storage target also includes an intermediate layer of light transparent insulating material having a dielectric constant greater than that of the phosphor material, said intermediate layer being provided on the support member between the phosphor layer and the target electrode.

low-velocity electrons and producing secondary electron emission to cause bistable storage of said charge image;

a collector electrode supported in contact with the dielectric layer on the opposite side of said dielectric layer from said target electrode to collect the secondary electrons; and

erase means for applying a voltage pulse to said target electrode to erase said charge image.

mg UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 4 607 Dated July 20 L 1971 lnventofls) Roger A.Frankland It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

f In the Abstract:

Line 11, "50" should be 5-- On column 2, line 53, "tub" should be -tube-.

( )n column 3, line 63, after "mesh" should be -electrode 1s extremely thin and has a fine mesh.

Signed and sealed this 22nd day of February 1972.

(SEAL) Attest:

EDWARD M.FLE'1CHER,JR. ROBERT GOTTSCHALK Attesting Officer- Gommissionerof Patents 

1. A storage apparatus capable of bistable storage of a charge image, comprising: a storage target including a support member of electrical insulative material, a storage dielectric layer of phosphor material provided on said support member, and a target electrode provided of said support member between the dielectric layer and said support member; writing means for bombarding the storage dielectric with a beam of high-velocity electrons to form a charge image thereon; holding means for bombarding the phosphor storage dielectric with low-velocity electrons and producing secondary electron emission to cause bistable storage of said charge image; and a collector electrode supported in contact with the phosphor dielectric layer on the opposite side of said dielectric layer from said target electrode so that said collector electrode is electrically insulated from said target electrode and positioned to collect the secondary electrons.
 2. Storage apparatus in accordance with claim 1 in which the storage dielectric layer contains phosphor material which emits a light image corresponding to the charge image, and the support member and target electrode are both light transparent.
 3. Storage apparatus in accordance with claim 2 in which the storage target also includes an intermediate layer of light transparent insulating mAterial having a dielectric constant greater than that of the phosphor material, said intermediate layer being provided on the support member between the phosphor layer and the target electrode.
 4. Storage apparatus in accordance with claim 1 in which the collector electrode is a mesh electrode formed by a coating of electrical conductive material on the phosphor layer.
 5. Storage apparatus in accordance with claim 2 in which the target electrode is divided into a plurality of spaced insulated conductive areas, and which also includes means for selectively applying erase pulses to different ones of said conductive areas to enable independent operation of the storage dielectric portions over said conductive areas in a storage mode or a nonstorage mode.
 6. Storage apparatus in accordance with claim 2 provided by a cathrode-ray storage tube in which the storage target and the collector electrode are contained within the evacuated envelope of said tube.
 7. Storage apparatus in accordance with claim 6 in which a flat glass faceplate portion of the envelope forms the support member of the storage target and is sealed to a ceramic funnel portion of the envelope by an intermediate seal portion, and said target electrode and said collector electrode are connected to external voltage sources outside of the envelope by spaced, insulated leads of electrical conductive material coated on the faceplate beneath the seal portion and extending completely through the seal.
 8. Storage apparatus in accordance with claim 7 in which the leads are made of tin oxide material and the seal portion is made of a crystallized glass material.
 9. Storage apparatus in accordance with claim 4 in which the storage dielectric is a continuous undivided layer of phosphor material.
 10. Storage apparatus in accordance with claim 9 in which the collector electrode is made of light-reflecting metal to increase the brightness of the light image.
 11. Storage apparatus in accordance with claim 1 which also includes electrical readout means for scanning the storage dielectric with an electron beam to produce an electrical readout signal of the charge image.
 12. Storage apparatus in accordance with claim 1 which also includes erase means for applying a voltage pulse to said target electrode to erase said charge image.
 13. Storage apparatus in accordance with claim 12 in which the storage dielectric layer contains phosphor material which emits a light image corresponding to the charge image, and the support member and target electrode are both light transparent.
 14. Storage apparatus in accordance with claim 13 in which the storage target also includes an intermediate layer of light transparent insulating material having a dielectric constant greater than that of the phosphor material, said intermediate layer being provided on the support member between the phosphor layer and the target electrode.
 15. A storage apparatus capable of bistable storage of a charge image, comprising: a storage target including a support member of electrical insulative material, a storage dielectric layer provided on said support member, and a target electrode provided on said support member beneath the dielectric layer; writing means for bombarding the storage dielectric with a beam of high-velocity electrons to form a charge image thereon; holding means for bombarding the storage dielectric with low-velocity electrons and producing secondary electron emission to cause bistable storage of said charge image; a collector electrode supported in contact with the dielectric layer on the opposite side of said dielectric layer from said target electrode to collect the secondary electrons; and erase means for applying a voltage pulse to said target electrode to erase said charge image. 